In so-called “second generation” biorefining, ethanol and/or other useful products are produced by fermentation of 5- and 6-carbons sugars derived from cellulose and hemicellulose chains of lignocellulosic biomass. In many “second generation” systems, fermentable sugars are produced through enzymatic hydrolysis of the biomass feedstock using commercially available cellulase enzyme preparations or enzyme-secreting microorganisms.
Effective enzymatic hydrolysis typically requires some form of pretreatment of the biomass feedstock in order to render cellulose chains more accessible. One particularly attractive category of pretreatment methods are so-called “hydrothermal” pretreatments. In these methods, pressurised and typically saturating steam is applied to the feedstock at temperatures between 150-220° C. corresponding to pressures between 4-23 bar, either with or without added acids, bases or other chemicals, in order to melt lignin and partially hydrolyse hemicellulose into soluble mono- and poly-saccharides, thereby improving accessibility of cellulose chains.
The capital cost of hydrothermal pretreatment systems is typically a significant component—as much as 20% or more—of total capital costs in a “second generation” biorefinery. Pressurized steam reactors suitable for large scale processing have proved expensive, in part because elaborate loading systems are required to process biomass feedstocks at ambient pressure. A variety of such loading systems have been reported, including “screw-plug” feeders, which compress biomass so as to form a pressure-tight seal while being introduced into the reactor. Other loading systems include elaborate sluice devices such as described in WO2011024145 as well as “low density” plug systems such as de-scribed in WO2010058285. Another significant component of capital cost for pressurised steam reactors is the requirement for internal agitation systems and mechanical means for continuous biomass transport.
Capital costs of the hydrothermal pretreatment system can be greatly reduced using embodiments of the vertical “plug flow” reactor described here. Isokinetic or “plug flow” transport of biomass through a pressurised steam reactor is driven by gravity alone. Effectively the only moving parts are mechanically simple “restrictor means,” the periodic motions of which permit a defined quantity of pretreated biomass to be removed from the lower part of the reactor at intervals providing a steady state input of feedstock and output of pretreated biomass.
Embodiments of the invention can be applied with particular advantage in processing biomass feedstock that has been previously subject to mechanical compression using a reciprocating piston press to bulk density of at least 500 kg/m3. As described in WO2013/152771, which is hereby expressly incorporated by reference in entirety, this manner of mechanical compression to this level of bulk density produces a mechanically induced vapour expansion or “steam explosion” which greatly alters the physical properties of the material. Most notably, the capacity of lignocellulosic feedstocks to adsorb water is greatly increased, both in terms of the rate at which adsorption occurs as well the total water holding capacity. For example, as described in WO2013/152771, wheat straw subjected to “mechanical steam explosion” can typically absorb between 5 and 10 times its own weight in water.
Using feedstocks that have been compressed in this manner, there is no need to presoak the material in order to achieve an appropriate water content during pretreatment. The compressed biomass very rapidly absorbs water within the pressurised reactor. Additional water content can easily be introduced at reactor temperature and pressure in order to control the amount of water absorbed within the reactor so as to reach an optimal water content during pretreatment. Thus, a relatively large quantity of “mechanically steam exploded,” compressed feedstock having low water content can be advantageously introduced into a pressurised steam reactor using a mechanically simple and inexpensive sluice loader. Furthermore, steam consumption during pretreatment can in this manner be greatly reduced. Biomass feedstocks are typically subject to hydrothermal pretreatment at water content between 65-80% by weight. See e.g. Larsen et al. (2008); Petersen et al. (2009); Kootstra et al. (2009); Larsen et al. (2012). Where hot water is added within the reactor to compressed biomass having low water content, wasteful heating of excess water is avoided. Also avoided is the unwanted increase in water content that occurs as a consequence of steam condensation associated with heating water content of the biomass feedstock up from ambient temperature.